1. Field of the Invention
This invention relates to a method for determining the flux angle of a rotating field machine upon starting, or respectively, a method for the position-oriented operation of a rotating field machine by means of a position transmitter which established the direction vector of a rotating reference system. The invention further relates to an apparatus for carrying out the method.
2. Description of the Prior Art
From DE-PS No. 21 32 178 a dynamic optimum control of a synchronous machine is known, in which a transmitter calculates from electrical parameters of the machine the stator-related components of a model vector describing the flux, the direct of which established a direction vector pointing in the direction of the field axis. A set value for the field-normal component of the stator current vector, taken off e.g. at the output of a speed governor, as well as a set value for the field-parallel component which indicates the magnetization to be supplied by the stator, determine a ("field-oriented") nominal vector--referred to this direction vector--for the stator current of the machine, from which by means of the direction vector a control vector is determined with which a (static) frequency converter is controlled and a corresponding stator current is impressed on the machine.
As a transmitter a so-called "voltage model" is often used which calculates the flux as an integral function of the EMF, i.e., as integral function of the voltage signal, less the ohmic and inductive voltage drops in the stator. At low speeds, however, the voltage model furnishes only an imprecise signal, and d-c voltage components occurring in the voltage measurements as faults, as well as integration errors in actual integrators lead to faulty determinations. Therefore, a field-oriented operation with a voltage model is possible only at higher frequencies. Besides, for the integration of the EMF the integration constant must be set by presetting a start value.
Instead of the flux direction it is possible in some cases to use as direction vector the axis of the EMF vector, which in the stationary case is rotated 90.degree. relative to the field axis. While the integration can then be eliminated, here too a sufficiently accurately controlled operation at low speeds is usually not possible. Instead it becomes necessary to find a different way to determine the direction vector.
A so-called "current model" simulates the processes by which flux is generated in the rotor, from instantaneous values of the current and the rotor position. For this, however, a mechanical pickup is needed for determining the rotor position. While this expense may be reduced by using an incremental speed sensing from another non-position-coded transmitter, as for instance a simple tachometer, at standstill or slow running of the machine the rotor axis must then be picked up ("located") in a different manner. This locating is often costly even with position-coded mechanical transmitters.
It is desirable to reduce the cost of determining the direction vector even when the operation of the synchronous machine is not exactly oriented on the field axis, but on the rotor axis, a high dynamic range being dispensed with. Thus, an approximate value for the armature reaction can be calculated for example from the set values of the stator current, in order to rotate the stator current vector relative to the rotor axis by a load-dependent nominal angle value which takes into consideration the angle between field axis and rotor axis. But then again only the determination of the field axis is reduced to a determination of the rotor axis; the difficulties of rotor localization at low speeds are not eliminated thereby. The approximate value for the armature reaction can be calculated for example from the set values of the stator current, in order to rotate the stator current vector relative to the rotor axis by a load-dependent nominal angle value which takes into consideration the angle between field axis and rotor axis. But then again only the ddetermination of the field axis is reduced to a determination of the rotor axis; the difficulties of rotor localization at low speeds are not eliminated thereby.
For machine tools, step motors and other areas of application, synchronous machines with a permanently excited rotor have proved successful. The armature reaction is negligible in such machines, so that rotor orientation becomes practically identical with field orientation and purely mechanical rotor position transmitters can be used as transmitter. However, here too, for correct starting the initial position or orientation of the rotor should be know, that is, for proper starting the rotor position transmitter must be set to the correct starting value.
This problem of rotor orientation thus occurs in particular in rotating field machines which have in the rotor definite preferential directions, including for instance reluctance machines. It can be solved in machines which have a separate exciter winding during standstill and calculating from the voltage induced in the stator the flux building up, which then points in the direction of the rotor axis. With permanent excitation, however, this possibility does not exist, nor would it solve the difficulties occurring at low speeds.
For field-oriented operation of asynchronous machines the voltage model is not feasible at the low speeds for position determination of the flux vector, while the current model shows the same problems linked with rotor localization, so that here too a costly mechanical transmitter may often be necessary.
Even non-field-oriented controls, such as a slip control or other characteristic-based controls, are usually based on the rotor position as the direction vector for the control of the stator current and require a mechanical position transmitter.
Lastly the invention is also largely independent of how the electric power supply to the machine is controlled. Thus, for example, when using an intermediate link converter, the intermediate circuit voltage can very well be impressed by a control voltage, and then e.g. a regulating and control unit can convert a set value for the stator current amplitude as command variable into the respective control signal for controlling the intermediate link voltage.
A special problem is the determination of the rotor position at start-up of a double-fed asynchronous machine. In this technology the rotor windings and the stator windings are connected in series, and the stator current flows also through the rotor. This takes place by means of slip rings, which also necessitate a change of the phase sequence. The stator current vector and its position relative to the stator is therefore equal to the rotor current vector and the relative position thereof to the rotor, the rotor rotating at double the stator frequency. Thus, with the determination of the rotor position also the position of the field vector is determined and vice versa. As a result, control is especially simple, in particular when field orientation is used.
If, without prior knowledge of the rotor position, a current is fed into such a double-fed asynchronous machine, the motor develops a torque not predictable as to magnitude and sign, and an uncontrolled movement of the rotor in one or the other direction must be reckoned with.